In recent years, for example, in the fields of auto parts, building material parts, furniture parts, etc., high strength sheet metal has been used for realizing lighter weight while securing rigidity. Such sheet metal has been shaped in the past by, for example, press forming or roll forming, but the drop in formability accompanying the increase in strength of the sheet metal is making it difficult to form such sheet metal into complicated shapes.
For example, when using press forming, a flange part of the shaped sheet metal upon which a tensile force acts sometimes becomes cracked at that flange part, or the flange part of the sheet metal upon which a compressive force acts sometimes becomes wrinkled at that flange part.
Further, when using roll forming, it is possible to form a sheet metal to a single simple cross-sectional shape in the longitudinal direction, but forming sheet metal into a complicated shape where the cross-section changes in the longitudinal direction is difficult.
On the other hand, it has been proposed to combine press forming and roll forming so as to shape sheet metal. For example, in PLT 1, there is provided a press apparatus for forming splines or other gearwheel shapes at the outer circumferential surface by roll forming which has a die at the outer circumferential surface of which the sheet metal to be worked is loaded, a die ring arranged concentrically with the die, and a plurality of forming rolls which are arranged radially at the inner circumference of the die ring and which rotate while being gripped between the sheet metal and the inner circumferential surface of the die ring. Further, the die ring is made to move relative to the die and the plurality of forming rolls are made to rotate so as to shape the sheet metal.
PLT 2 discloses a roll forming method in a roll forming apparatus which forms a long object like a frame part into a curved shape, wherein the forming rolls are made to move on a translation cam matching the shape of the long object and wherein the forming rolls are made to ascend or descend so as to follow the shape of the translation cam and thereby work the object into a shape curved in the longitudinal direction.
PLT 3 discloses a roll forming method which joins together perpendicularly intersecting sheet metal (seaming) during which folding back the sheet metal at the seam part (hemming) and forming the seam part at that time by placing a forming roll having a right angled cross-section against the right angle seam part of the sheet metal and rotating it while pressing.